1. Field of the Invention
The present invention relates to the recovery of metal values from refractory sulfide and refractory carbonaceous sulfide ores. More particularly, the present invention relates to the heap biooxidation of refractory sulfide ores using a recycled bioleachate solution.
2. Description of the Prior Art
Gold is one of the rarest metals on earth. Gold ores can be categorized into two types: free milling and refractory. Free milling ores are those that can be processed by simple gravity techniques or direct cyanidation. Refractory ores, on the other hand, are not amenable to conventional cyanidation treatment. Gold bearing deposits are deemed refractory if they cannot be economically processed using conventional cyanide leaching techniques because insufficient gold is solubilized. Such ores are often refractory because of their excessive content of metallic sulfides (e.g., pyrite and arsenopyrite) and/or organic carbonaceous matter.
A large number of refractory ores consist of ores with a precious metal such as gold occluded in iron sulfide particles. The iron sulfide particles consist principally of pyrite and arsenopyrite. If the gold, or other precious metal, remains occluded within the sulfide host, even after grinding, then the sulfides must be oxidized to liberate the encapsulated precious metal values and make them amenable to a leaching agent (or lixiviant); thus, the sulfide oxidation process reduces the refractory nature of the ore.
A number of processes for oxidizing the sulfide minerals to liberate the precious metal values are well known in the art. These methods can generally be broken down into two types: mill operations and heap operations.
Mill operations are typically expensive processes having high operating and capital costs. As a result, even though the overall recovery rate is typically higher for mill type processes, mill operations are not applicable to low grade ores, that is ores having a gold concentration less than approximately 0.07 oz/ton, and even as low as approximately 0.02 oz/ton.
Two well known methods of oxidizing sulfides in mill type operations are pressure oxidation in an autoclave and roasting.
Oxidation of sulfides in refractory gold ores can also be accomplished using acidophilic, autotrophic microorganisms, such as Thiobacillus ferrooxidans, Sulfolobus, Acidianus species and facultative-thermophilic bacteria in a microbial pretreatment. These microorganisms can utilize the oxidation of sulfide minerals as an energy source during metabolism. During the oxidation process, the foregoing microorganisms oxidize the iron sulfide particles to cause the solubilization of iron as ferric iron, and sulfide, as sulfate ion.
Oxidation of refractory sulfide ores using microorganisms, or, as often referred to, biooxidation, can be accomplished in a mill process or a heap process. Compared to pressure oxidation and roasting, biooxidation processes are simpler to operate, require less capital, and have lower operating costs. Indeed, biooxidation is often chosen as the process for oxidizing sulfide minerals in refractory sulfide ores because it is economically favored over other means to oxidize the ore. However, because of the slower oxidation rates associated with microorganisms when compared to chemical and mechanical means to oxidize sulfide refractory ores, biooxidation is often the limiting step in the mining process.
One mill type biooxidation process involves comminution of the ore followed by treating a slurry of the ore in a bioreactor where microorganisms can use the finely ground sulfides as an energy source. Such a mill process was used on a commercial scale at the Tonkin Springs mine. However, the mining industry has generally considered the Tonkin Springs biooxidation operation a failure. A second mill type biooxidation process involves separating the gold bearing sulfides from the ore using conventional sulfide concentrating technologies, such as flotation, and then oxidizing the sulfides in a stirred bioreactor to alleviate their refractory nature. Commercial operations of this type are in use in Africa, South America and Australia.
Biooxidation in a heap process typically entails forming a heap of refractory sulfide ore particles and then inoculating the heap with a microorganism capable of biooxidizing the sulfide minerals in the ore. After biooxidation has come to a desired end point, the heap is drained and washed out by repeated flushing. The liberated precious metal values are now ready to be leached with a suitable lixiviant. Typically precious metal containing ores are leached with cyanide because it is the most efficient leachant or lixiviant for the recovery of the precious metal values from the ore. However, if cyanide is used as the lixiviant, the heap must first be neutralized.
Because biooxidation occurs at a low, acidic pH while cyanide processing must occur at a high, basic pH, heap biooxidation followed by conventional cyanide processing is inherently a two step process. As a result, processing options utilizing heap biooxidation must separate the two steps of the process. This is conventionally done by separating the steps temporally. For example, in heap biooxidation, the heap is first biooxidized and then rinsed, neutralized and treated with cyanide. To accomplish this economically and practically, most heap biooxidation operations use a permanent heap pad in one of several ore on--ore off configurations.
Of the various biooxidation processes available, heap biooxidation has the lowest operating and capital costs. This makes heap biooxidation processes particularly applicable to low grade or waste type ores, that is ores having a gold (or equivalent precious metal value) concentration of less than about 0.07 oz/ton. Heap biooxidation, however, has very slow kinetics compared to mill biooxidation processes. Heap biooxidation can require many months in order to sufficiently oxidize the sulfide minerals in the ore to permit gold or other precious metal values to be recovered in sufficient quantities by subsequent cyanide leaching for the process to be considered economical. Heap biooxidation operations, therefore, become limited by the length of time required for sufficient biooxidation to occur to permit the economical recovery of gold. The longer the time required for biooxidation the larger the permanent pad facilities and the larger the necessary capital investment. At mine sites where the amount of land suitable for heap pad construction is limited, the size of the permanent pad can become a limiting factor in the amount of ore processed at the mine and thus the profitability of the mine. In such circumstances, rate limiting conditions of the biooxidation process become even more important.
The rate limiting conditions of the heap biooxidation process include inoculant access, nutrient access, air or oxygen access, and carbon dioxide access, which are required to make the process more efficient and thus an attractive treatment option. Moreover, for biooxidation, the induction times concerning biooxidants, the growth cycles, the biocide activities, viability of the bacteria and the like are important considerations because the variables such as accessibility, particle size, settling, compaction and the like are economically irreversible once a heap has been constructed. As a result, heaps cannot be repaired once formed, except on a limited basis.
The methods disclosed in U.S. Pat. No. 5,246,486, issued Sep. 21, 1993, and U.S. Pat. No. 5,431,717, issued Jul. 11, 1995, by one of the above named inventors, both of which are hereby incorporated by reference, are directed to increasing the efficiency of the heap biooxidation process by ensuring good fluid flow (both gas and liquid) throughout the heap.
Solution inventory and solution management, however, also pose important rate limiting considerations for heap biooxidation processes. The solution drained from the biooxidation heap will be acidic and contain bacteria and ferric ions. Therefore, this solution can be used advantageously in the agglomeration of new ore or by recycling it back to the top of the heap. However, toxic or inhibitory materials can build up in this off solution. For example, ferric ions, which are generally a useful aid in pyrite leaching, are inhibitory to bacteria growth when their concentration exceeds about 30 g/l. Biocidically active metals can also build-up in this solution, retarding the biooxidation process. Biocidically active metals that are often found in refractory sulfide ores include arsenic, antimony, cadmium, lead, mercury, and molybdenum. Other toxic metals, biooxidation byproducts, dissolved salts and bacterially produced material can also be inhibitory to the biooxidation rate. When these inhibitory materials build up in the off solution to a sufficient level, recycling of the off solution becomes detrimental to the rate at which the biooxidation process proceeds. Indeed, continued recycling of an off solution having a sufficient build-up of inhibitory materials will stop the biooxidation process altogether.
In the past, to prevent excessive build-up of inhibitory materials in the bioleachate off solution collected from the heap, mine operations have simply replaced, or diluted, the effluent from the heap with fresh inoculant solution. This is expensive as it increases the consumption of fresh water and also increases the need for waste water treatment.
A method is disclosed in U.S. Pat. No. 5,246,486, for removing inhibitory concentrations of arsenic or iron from the heap off solution, which are defined in that reference as concentrations exceeding about 14 g/l and 30 g/l, respectively. The method disclosed in this patent entails raising the pH of the bioleachate off solution to above 3 so that the arsenic ions in solution coprecipitate with ferric ions in solution. There are, however, several inadequacies with the process disclosed in this patent. First, as described above, there are a multitude of potential inhibitory materials that can be leached from the ore or that can be formed as a result of the bioleaching process; thus, simply monitoring the arsenic or ferric ion build-up in the bioleachate off solution will not alleviate the problem of inhibitory concentrations of other metals or materials from building up in the off solution. Furthermore, the off solution in most instances will not contain inhibitory concentrations of any one specific inhibitory material. Nonetheless, the biooxidation process will be retarded from the build-up of a combination of a number of inhibitory materials in the recycled off solution. Therefore, the combined concentration of at least two inhibitory materials may be sufficient to inhibit the biooxidation rate of refractory sulfide ore particles in the heap even though the concentration of no single material is above its inhibitory concentration.
Consequently, a need exists in heap biooxidation processes for a method of removing inhibitory concentrations of a group of inhibitory materials within the heap off solution. Such a method would reduce the time required for heap biooxidation processes and concomitantly reduce the capital required for constructing the heap biooxidation facility. In addition, such a method would reduce the constriction heap biooxidation typically places on mine operations.